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PAIN
A complex subjective phenomenon made up of a sensation indicating real or potential tissue damage and the affective response this generates.
Classification
Classifying pain is helpful when organizing a therapeutic approach; many schema are possible. First is the distinction between acute and chronic pain. Acute pain, an essential biologic signal of the potential for or the extent of injury, is usually short-lived; it is associated with hyperactivity of the sympathetic nervous system (eg, tachycardia, increased respiratory rate and BP, diaphoresis, and dilated pupils). The concurrent affect is anxiety. Treatment involves removing the underlying cause, if possible; the pain is usually readily ameliorated with analgesics.
Chronic pain is usually defined as pain persisting >3 to 6 mo, though the characteristic features can occur earlier or later than this arbitrary period. Pain of this duration loses its adaptive biologic role. Vegetative signs gradually develop (eg, lassitude, sleep disturbance, decreased appetite, loss of taste for food, weight loss, diminished libido, and constipation). A depressed affect predominates. In many patients, organic disease may be absent or insufficient to explain the degree of pain. In these patients and in many with organic disease, the psychologic factors become the primary contributor to impairment. Therapy is often difficult and prognosis is guarded.
Chronic pain states: One broad classification distinguishes somatogenic pains, those explicable in terms of physiologic mechanisms, from psychogenic pains, those better understood in psychologic terms. A related taxonomy attempts to further distinguish pains by their presumed pathogenesis. Nociceptive pain is pain that is judged to be commensurate with ongoing activation of pain-sensitive nerve fibers, either somatic or visceral. When somatic nerves are involved, the pain is typically experienced as aching or pressure-like (eg, most instances of cancer pain). Neuropathic pain is due to nerve tissue damage. The chronic pain that results may be dependent on the efferent function of the sympathetic nervous system (sympathetic-mediated pain) or may primarily involve either ongoing peripheral pathology (eg, nerve compression or neuroma formation) or CNS changes (deafferentation pain). This chronic neuropathic pain is discussed further below. Finally, psychogenic pains occur without an organic lesion sufficient to explain the degree of pain and disability.
Specific pain syndromes may have a multifactorial etiology; eg, most cancer pain syndromes have a prominent nociceptive component, but may also include deafferentation states caused by nerve damage from tumor or its treatment, and psychologic processes related to loss of function and fear of disease progression. Typically, nociceptive pain also occurs in patients with pain syndromes related to arthritis, sickle cell disease, and hemophilia. Pain evaluation is basic to its management, including an ongoing assessment of the treatability of the underlying lesion. A distinction between continuous and recurrent acute pain (as in sickle cell disease) also is important, since the treatment plans will differ.
Damage to the nervous system can produce a variety of chronic pain syndromes; some are peripheral (eg, compression neuropathies or neuroma formation) and some are due to changes in central pain pathways resulting in deafferentation pain syndromes. Pain in which psychologic processes predominate is also common, and many pain syndromes (eg, many patients with failed low back, atypical facial, and chronic pelvic pain) can be included in this category. Management of these diverse patients has much in common.
Several specific pain syndromes are difficult to classify; eg, myofascial pain syndrome (also called fibromyositis) is presumably due to chronic injury to muscle and surrounding connective tissue. Its prevalence is unknown and it is often misdiagnosed. The sine qua non is the identification of trigger points in muscle; pain can be managed by inactivation of these by injection or careful stretching. Chronic headache is also difficult to classify pathogenically and in most patients probably involves a complex interaction between nociceptive perturbations in muscles and blood vessels, and psychologic factors.
Evaluation of Patients with Pain
Though the etiology of acute or chronic pain varies remarkably, the rigorous evaluation each patient deserves has many common elements. In all cases, a detailed history of pain should assess severity, location, quality, duration and course, timing (including frequency of remissions and degree of fluctuation), exacerbating and relieving factors, and associated symptoms (with emphasis on psychologic state and vegetative symptoms). Drug use, its efficacy, and adverse effects should be queried and other treatments detailed. Ongoing litigation should be identified. A previous personal or family history of chronic pain can often illuminate the current problem and should be evaluated. Finally, a detailed assessment of the patient's level of function is necessary. This should focus on family relationships (including sexual), social network, and employment or avocations. In all spheres, the history should attempt to reveal the role played by the patient's pain in his or her interactions with others and attempts at normal living. Through this comprehensive interview, the issue of secondary gain is assessed, an evaluation is made of current and premorbid psychopathology, the role of family pathology is clarified, and a sense of the overall degree of abnormal illness behavior is obtained.
The pain history should also try to identify the meaning to the patient of the pain. It is more socially acceptable to report pain than anxiety or depression, and proper therapy often depends on sorting out these similarly described but divergently experienced perceptions. Similarly, the distinction between pain and suffering should be clarified. This is especially salient in the cancer patient, whose suffering may be due as much to loss of function and fear of impending death as to pain.
Physical examination helps identify underlying causes and further evaluates the degree of functional impairment. Laboratory and x-ray examinations should be obtained as warranted. Organic etiologies always should be vigorously sought, since pain is managed best by removing the underlying cause; the likelihood of a prominent psychologic contribution to the pain should not preclude this search. The corollary is also true: Once an organic explanation has been appropriately ruled out, the patient is ill-served by additional useless tests. The sense of progress they provide for both physician and patient is illusory, actually perpetuating maladaptive behaviors and impeding a return to more normal function.
Regardless of the underlying disease mechanism, pain management depends on the understanding that the patient's perception of pain can represent more than the pathology intrinsic to the disease. Although the specifics of management vary remarkably from patient to patient, the principles discussed below can be applied to all with acute or chronic pain, including those with known painful medical disease whose pain fails to respond to the usual measures or whose level of disability appears out of proportion to the degree of impairment.
OPIOID ANALGESICS AND ANTAGONISTS
TREATMENT OF ACUTE POSTOPERATIVE PAIN
TREATMENT OF CANCER PAIN
NEUROPATHIC PAIN
PSYCHOGENIC PAIN SYNDROMES
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